[Baggage] Simplify Get APIs

[utpilla]

Changes

• We can loosen the restrictions on the type of key required for get baggage APIs to any type that is guaranteed to return a &str
• Currently, we require the type to be Into<Key> which not only requires the user to know which types can be converted into Key but also, we don't support conversions into Key for a lot of types such as Rc<String>, RefCell<String> etc.

Please provide a brief description of the changes here.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 75.0%. Comparing base (d5c86d9) to head (a30ef98).

Files	Patch %	Lines
opentelemetry/src/baggage.rs	50.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1902     +/-   ##
=======================================
- Coverage   75.0%   75.0%   -0.1%     
=======================================
  Files        122     122             
  Lines      20279   20285      +6     
=======================================
+ Hits       15211   15215      +4     
- Misses      5068    5070      +2     

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